For cellular Orthogonal Frequency Division Multiple Access (OFDMA) systems with an aggressive frequency reuse plan, using equal power for all subcarriers in the downlink transmission may lead to severe inter-cell or inter-sector interference. This may then produce undesirable results, such as outages and low system throughput. Individualized power control that is tailored to each specific user can reduce interference, but may use a significant portion of the system bandwidth for power control. This may also reduce system throughput.
With an equal downlink power system, all subcarriers are allocated with equal power. Near cell boundaries, users may experience multi-cell interference, because users may be receiving subcarriers, that were transmitted with equal power, from different base stations at near-equal distance. For a user operating with a particular subcarrier just inside the boundary of one cell, the interfering signal from the base station in an adjacent cell may be nearly as strong as the desired signal. This user may then experience a fairly low signal-to-interference ratio (SIR), which could result in poor quality service, inefficient bandwidth usage or an outage.
Users close to a base station, however, are likely to receive a relatively strong signal which provides a high SIR. Often, the SIR may be higher than is necessary in order to achieve acceptable performance. In some cases, the SIR is high enough that some of the transmitter power may be wasted on diminishing performance gains. An imbalance in SIRs across users, that may range from unacceptably low to potentially wasteful, will hamper the overall system efficiency and throughput.
Alternatively, downlink power control that custom-tailors transmit power based on specific user needs can maintain the range of SIRs across users within a more narrow range of values. Thus, a user nearby a base station may not receive a high power transmission, but rather only enough power to maintain an acceptable SIR. Since the subcarrier allocated to the nearby user may be transmitted with less power than in an equal power system, that particular subcarrier is less likely to interfere with a user in a neighboring cell or sector who is using the same subcarrier.
Unfortunately, in a custom-tailored power system, each user may often need to communicate power control information back to the serving base station in a closed loop configuration. This communication requirement will use system bandwidth for each user, affecting overall system efficiency and throughput. Additionally, the base station may have an additional computational burden attempting to tailor power levels for each user.